1. Field of the Invention
The present invention relates to a photoelectric conversion device and imaging system.
2. Description of the Related Art
Recently, photoelectric conversion devices having an amplification function, especially CMOS photoelectric conversion devices, have attracted attention. When very strong light (bright light) such as solar light enters the CMOS photoelectric conversion device, the gray level of the output voltage of a pixel irradiated with the strong light abruptly drops, and the gray level of the pixel may decrease to black level. This phenomenon will be called a high-brightness darkening phenomenon.
The high-brightness darkening phenomenon occurs when performing correlated double sampling (CDS) processing. This phenomenon arises from charges (signal charges) overflowing from a photoelectric conversion unit upon irradiation with strong light during a period in which a noise-level signal is output. More specifically, for example, when reading out a noise-level signal from the detection node (floating diffusion) of a pixel, some of many charges generated in the photoelectric conversion unit (photodiode) leak to the detection node of the pixel. If many charges enter the detection node, a noise-level voltage becomes lower than an (ideal) reset-level voltage and comes close to an optical signal-level voltage. In this case, if an image signal is obtained by CDS processing of calculating the difference between a noise-level voltage and an optical signal-level voltage, the luminance component of the image signal is calculated to be smaller than an original luminance component. This is the mechanism of the high-brightness darkening phenomenon. The above-mentioned voltage relationship assumes that electrons are used as signal charges and an NMOS transistor is used as an amplification MOS transistor for reading out a signal to a signal line. The voltage change direction is reversed when holes are used as signal charges or a PMOS transistor is used as an amplification MOS transistor. The high-brightness darkening phenomenon raises a noise-level voltage.
As a conventional photoelectric conversion device considering the high-brightness darkening phenomenon, a photoelectric conversion device disclosed in Japanese Patent Laid-Open No. 2005-57612 is known. As shown in FIG. 6, the photoelectric conversion device disclosed in Japanese Patent Laid-Open No. 2005-57612 includes a pixel 4, detection unit 2, and second holding means 3.
The detection unit 2 includes a detection means, a first holding means, a feedback means, and a clipping means for clipping an electric potential of a signal line SIG in accordance with a fed-back voltage. The detection means detects an electric potential of the signal line SIG. The first holding means holds a voltage based on the electric potential detected by the detection means. The feedback means feeds back a voltage held by the first holding means to the clipping means. The clipping means clips the electric potential of the signal line SIG in accordance with the fed-back voltage by the feedback means.
In the photoelectric conversion device disclosed in Japanese Patent Laid-Open No. 2005-57612, the electric potential of the signal line SIG that is detected by the detection means is held by the first holding means, and then fed back by the feedback means to the clipping means. For this reason, the difference between the electric potential of the signal line SIG and the input electric potential of the clipping means cannot be arbitrarily set. More specifically, the clipping means is formed from an NMOS transistor. An electric potential (clipping electric potential) clipped by the NMOS transistor is subject to the restriction that the clipping electric potential has to be lower than the electric potential of the signal line SIG by the threshold voltage of the NMOS transistor.
To effectively suppress generation of the high-brightness darkening phenomenon, the clipping electric potential is preferably as close as possible to a reset level which should be transferred via the signal line SIG while the detection node of a pixel is reset. However, in the photoelectric conversion device disclosed in Japanese Patent Laid-Open No. 2005-57612, the clipping electric potential is subject to the restriction that the clipping electric potential has to be lower than the reset level by the threshold voltage of the NMOS transistor. As a result, it becomes difficult to effectively suppress generation of the high-brightness darkening phenomenon.